What age are these books suitable for...

My dd1 is 6 and I'm wondering if any of the above would be suitable for me to read to her. (I know she couldn't read them herself yet). I have fond memories of these stories from my own childhood but can't remember them in great detail so I'm not sure if dd1 is old enough to understand/appreciate them.

Not Marianne Dreams at all - 9 for that I'd say (or older). If you can't quite remember, it's a spooky fable of a book, about two children recovering from very serious illness (the boy has had polio) and they meet via dreams and drawings in a sort of out-of-body psychic space where they fight against some incredibly scary beings. (Who are, of course, the part of their own selves which do not want them to try to get better.) It's heavy. Read it again yourself!

Tom's Midnight Garden is much cosier.

Charlotte Sometimes is cosier, too, but I think it is worth saving till she can read it herself - maybe 8 for that.